X-rays reveal hidden leg of an ancient snake

Synchrotron X-ray investigation of a fossilised snake with legs is helping scientists to better understand how in the course of evolution snakes have lost their legs, and whether they evolved from terrestrial lizards or from reptiles living in the oceans. New 3-D X-ray images reveal that the internal architecture of the ancient snake’s leg bones resemble those of modern terrestrial lizard legs. The results are published on 8 February in the Journal of Vertebrate Paleontology.

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A novel X-ray imaging technology is helping scientists to better understand how in the course of evolution snakes have lost their legs. The researchers hope that the new data will help to resolve a heated debate about the origin of snakes: whether they evolved from a terrestrial lizard or from one that lived in the oceans. New, detailed 3-D images reveal that the internal architecture of an ancient snake’s leg bones strongly resembles those of modern terrestrial lizard legs. The results are published in the 8 February issue of the Journal of Vertebrate Paleontology.

The team of researchers was led by Alexandra Houssaye from Museum National d'Histoire Naturelle (MNHN) and CNRS in Paris, France, and included scientists from the European Synchrotron Radiation Facility (ESRF) in Grenoble, France, where the X-ray imaging was performed, and the Karlsruhe Institute of Technology (KIT), Germany, where a sophisticated technique and a dedicated instrument to take the images were developed.

Only three specimens exist of fossilised snakes with preserved leg bones. Eupodophis descouensi, the ancient snake studied in this experiment, was discovered ten years ago in 95-million-year-old rocks in Lebanon. About 50 cm long overall, it exhibits a small leg, about 2 cm long, attached to the animal’s pelvis. This fossil is key to understanding the evolution of snakes, as it represents an intermediate evolutionary stage when ancient snakes had not yet completely lost the legs they inherited from earlier lizards. Although the fossil exhibits just one leg on its surface, a second leg was thought to be concealed in the stone, and indeed this leg was revealed in full detail thanks to synchrotron X-rays.

Photograph of Eupodophis descouensi, a fossil snake from the Cretaceous Period (95 million years ago) of Lebanon. The scale bar equals 1 cm. Credit: A. Houssaye.

The high-resolution 3-D images, in particular the fine details of the buried small leg, suggest that this species lost its legs because they grew more slowly, or for a shorter period of time. The data also reveal that the hidden leg is bent at the knee and has four ankle bones but no foot or toe bones.

"The revelation of the inner structure of Eupodophis hind limbs enables us to investigate the process of limb regression in snake evolution," says Alexandra Houssaye.

The scientists used synchrotron laminography, a recent imaging technique specially developed for studying large, flat samples. It is similar to the computed tomography (CT) technique used in many hospitals, but uses a coherent synchrotron X-ray beam to resolve details a few micrometres in size — some 1000 times smaller than a hospital CT scanner. For the new technique, the fossil is rotated at a tilted angle in a brilliant high-energy X-ray beam, with thousands of two-dimensional images recorded as it makes a full 360-degree turn. From these individual images, a high-resolution, 3-D representation is reconstructed, which shows hidden details like the internal structure(s) of the legs.

"Synchrotrons, these enormous machines, allow us to see microscopic details in fossils invisible to any other techniques without damage to such invaluable specimens," says Paul Tafforeau of the ESRF, a co-author of the study.

Synchrotron X-ray investigation of a fossilised snake with legs is helping scientists better understand how in the course of evolution snakes have lost their legs, and whether they evolved from terrestrial lizards or from reptiles living in the oceans. New 3-D X-ray images reveal the internal architecture of the ancient snake's leg bones resemble those of modern terrestrial lizard legs.

Top image: 3-D model of the previously hidden second leg of Eupodophis. The bones are articially colored to highlight the internal structure of the bone and show how the snake's leg grew. Scale bar at bottom right = 0.5 mm, Credit A. Houssaye.